The influence of knee angle on human quadriceps femoris performance

Haan, A. de [Promotor]Ruiter, C.J. de [Copromotor

Mechanical properties of Ti-6Al-4V selectively laser melted parts with body-centred-cubic lattices of varying cell size

Significant weight savings in parts can be made through the use of additive manufacture (AM), a process which enables the construction of more complex geometries, such as functionally graded lattices, than can be achieved conventionally. The existing framework describing the mechanical properties of lattices places strong emphasis on one property, the relative density of the repeating cells, but there are other properties to consider if lattices are to be used effectively. In this work, we explore the effects of cell size and number of cells, attempting to construct more complete models for the mechanical performance of lattices. This was achieved by examining the modulus and ultimate tensile strength of latticed tensile specimens with a range of unit cell sizes and fixed relative density. Understanding how these mechanical properties depend upon the lattice design variables is crucial for the development of design tools, such as finite element methods, that deliver the best performance from AM latticed parts. We observed significant reductions in modulus and strength with increasing cell size, and these reductions cannot be explained by increasing strut porosity as has previously been suggested. We obtained power law relationships for the mechanical properties of the latticed specimens as a function of cell size, which are similar in form to the existing laws for the relative density dependence. These can be used to predict the properties of latticed column structures comprised of body-centred-cubic (BCC) cells, and may also be adapted for other part geometries. In addition, we propose a novel way to analyse the tensile modulus data, which considers a relative lattice cell size rather than an absolute size. This may lead to more general models for the mechanical properties of lattice structures, applicable to parts of varying size

Muscle activation and blood flow do not explain the muscle length-dependent variation in quadriceps isometric endurance

We investigated the role of central activation in muscle length-dependent endurance. Central activation ratio (CAR) and rectified surface electromyogram (EMG) were studied during fatigue of isometric contractions of the knee extensors at 30 and 90° knee angles (full extension = 0°). Subjects (n = 8) were tested on a custom-built ergometer. Maximal voluntary isometric knee extension with supramaximal superimposed burst stimulation (three 100-μs pulses; 300 Hz) was performed to assess CAR and maximal torque capacity (MTC). Surface EMG signals were obtained from vastus lateralis and rectus femoris muscles. At each angle, intermittent (15 s on 6 s off) isometric exercise at 50% MTC with superimposed stimulation was performed to exhaustion. During the fatigue task, a sphygmomanometer cuff around the upper thigh ensured full occlusion (400 mmHg) of the blood supply to the knee extensors. At least 2 days separated fatigue tests. MTC was not different between knee angles (30°: 229.6 ± 39.3 N·m vs. 90°: 215.7 ± 13.2 N·m). Endurance times, however, were significantly longer (P < 0.05) at 30 vs. 90° (87.8 ± 18.7 vs. 54.9 ± 12.1 s, respectively) despite the CAR not differing between angles at torque failure (30°: 0.95 ± 0.05 vs. 90°: 0.96 ± 0.03) and full occlusion of blood supply to the knee extensors. Furthermore, rectified surface EMG values of the vastus lateralis (normalized to prefatigue maximum) were also similar at torque failure (30°: 56.5 ± 12.5% vs. 90°: 58.3 ± 15.2%), whereas rectus femoris EMG activity was lower at 30° (44.3 ± 12.4%) vs. 90° (69.5 ± 25.3%). We conclude that differences in endurance at different knee angles do not find their origin in differences in central activation and blood flow but may be a consequence of muscle length-related differences in metabolic cost

Knee angle-dependent oxygen consumption of human quadriceps muscles during maximal voluntary and electrically evoked contractions

Fatigability and muscle oxygen consumption (mVO2) during sustained voluntary isometric knee extensions are less at extended (30° knee angle; 0°, full extension) versus flexed knee angles (90°). This lower energy consumption may partially result from lower neural activation at extended knee angles. We hypothesized a smaller difference in mVO2 between extended and flexed knee angles during electrical stimulation, which guaranteed maximal activation, than during maximal voluntary contractions (MVC). In eight healthy young males, MVC extension torque was obtained at 30°, 60° and 90° knee angles. mVO2 of the rectus femoris (RF), vastus lateralis (VL) and medialis muscle was measured using near-infrared spectroscopy during tetanic (10 s) and maximal voluntary (15 s) contractions (MVC15). For electrically induced contractions, steady state mVO2 was reached at similar (P > 0.05) times after torque onset (4.6 ± 0.7 s) at all knee angles. In contrast, during MVC15 at 30° mVO2 was reached at 7.1 ± 1.1 s, significantly later compared to 60° and 90° knee angles. The knee angle dependent differences in mVO2 were not lower in electrically induced contractions (as hypothesised) but were similar as in voluntary contractions. Normalized mVO2 at 30° (percentage 90° knee angle) was 79.0 ± 9.4% (across muscles) for electrically induced and 79.5 ± 7.6% (across muscles) for voluntary contractions (P < 0.05). We conclude that the slower onset of mVO2 during voluntary effort at 30° may have been due to a lower maximal activation. However, because steady state mVO2 both during electrically induced and voluntary contractions was ~20% less at extended versus flexed knee angles, the causes for the lower mVO2 must reside within the muscle itself

Initial phase of maximal voluntary and electrically stimulated knee extension torque development at different knee angles

We investigated the capacity for torque development and muscle activation at the onset of fast voluntary isometric knee extensions at 30, 60, and 90° knee angle. Experiments were performed in subjects (n = 7) who had high levels (>90%) of activation at the plateau of maximal voluntary contractions. During maximal electrical nerve stimulation (8 pulses at 300 Hz), the maximal rate of torque development (MRTD) and torque time integral over the first 40 ms (TTI40) changed in proportion with torque at the different knee angles (highest values at 60°). At each knee angle, voluntary MRTD and stimulated MRTD were similar (P < 0.05), but time to voluntary MRTD was significantly longer. Voluntary TTI40 was independent (P > 0.05) of knee angle and on average (all subjects and angles) only 40% of stimulated TTI40. However, among subjects, the averaged (across knee angles) values ranged from 10.3 ± 3.1 to 83.3 ± 3.2% and were positively related (

Rosuvastatin reduces atherosclerosis development beyond and independent of its plasma cholesterol-lowering effect in APOE*3-Leiden transgenic mice: Evidence for antiinflammatory effects of rosuvastatin

Background - Statins can exert anti-inflammatory antiatherosclerotic effects through an anti-inflammatory action, independent of lowering cholesterol. We addressed the question whether the anti-inflammatory activities of statins can reduce atherosclerosis beyond the reduction achieved by cholesterol lowering per se. Methods and Results - Two groups of 20 female APOE*3-Leiden mice received either a high-cholesterol diet (HC) or a high-cholesterol diet supplemented with 0.005% (wt/wt) rosuvastatin (HC+R). The HC diet alone resulted in a plasma cholesterol concentration of 18.9±1.4 mmol/L, and administration of rosuvastatin lowered plasma cholesterol to 14.1±0.7 mmol/L. In a separate low-cholesterol (LC) control group, the dietary cholesterol intake was reduced, which resulted in plasma cholesterol levels that were comparable to the HC+R group (13.4±0.8 mmol/L). Atherosclerosis in the aortic root area was quantified after 24 weeks. As compared with the HC group, the LC group had a 62% (P<0.001) reduction in cross-sectional lesion area. When compared with the LC group, the HC+R group showed a further decrease in cross-sectional lesion area (80%, P<0.001), size of individual lesions (63%, P<0.05), lesion number (58%, P<0.001), monocyte adherence (24%, P<0.05), and macrophage-containing area (60%, P<0.001). Furthermore, rosuvastatin specifically suppressed the expression of the inflammation parameters MCP-1 and TNF-α in the vessel wall and lowered plasma concentrations of serum amyloid A and fibrinogen, independent of its cholesterol-lowering effect. Conclusions - Rosuvastatin reduces atherosclerosis beyond and independent of the reduction achieved by cholesterol lowering alone. This additional beneficial effect of rosuvastatin may be explained, at least partly, by its antiinflammatory activity. Chemicals/CAS: amyloid A protein, 59165-71-8; fibrinogen, 9001-32-5; rosuvastatin, 147098-18-8, 147098-20-2; cholesterol, 57-88-5; fluorobenzene, 2367-82-0, 327-54-8, 363-72-4, 367-23-7, 372-38-3, 462-06-6, 540-36-3; lipid, 66455-18-3; Anti-Inflammatory Agents; apolipoprotein E3 (Leidein); Apolipoprotein E3; Apolipoproteins E; Cholesterol, 57-88-5; Cytokines; Fluorobenzenes; Hydroxymethylglutaryl-CoA Reductase Inhibitors; Lipids; Lipoproteins; Pyrimidines; rosuvastatin, 287714-41-4; Sulfonamide

Transgenic flavonoid tomato intake reduces C-reactive protein in human C-reactive protein transgenic mice more than wild-type tomato

The increased consumption of fruits and vegetables is associated with reduced cardiovascular disease. The molecular basis of this health effect is not fully understood, yet dietary flavonoids are thought to play an important role. Genetic engineering has enabled us to overexpress specific flavonoids (flavones and flavonols) in tomato fruit. Human C-reactive protein transgenic (CRPtg) mice express markers of cardiovascular risk that allow us to study of the putative health effects of wild-type tomato (wtTom) and flavonoid-enriched tomato (flTom). In this study, we analyzed whether consumption of wtTom, at a dose achievable with a human diet, has beneficial effects on cardiovascular risk markers and whether flTom may enhance such effects. CRPtg mice were fed a diet containing 4 g/kg wtTom, flTom peel, vehicle, or 1 g/kg fenofibrate, which reportedly reduces cardiovascular risk, for 7 wk. Markers of general health (bodyweight, food intake, and plasma alanine aminotransferase activities) and of cardiovascular risk (plasma CRP, fibrinogen, E-selectin, and cholesterol levels) were analyzed. All groups had comparable food intakes and body-weight gains. Plasma alanine aminotransferase activities increased significantly in vehicle and fenofibrate-treated mice. Compared with baseline, wtTom and flTom significantly reduced basal human CRP concentrations by 43 and 56%, respectively. The CRP-lowering effect of flTom significantly exceeded that of wtTom. The effects of flTom on CRP were reversed within a 2-wk washout period. WtTom and flTom did not affect fibrinogen, but comparably repressed E-selectin expression and upregulated HDL cholesterol. Tomato peel consumption improved cardiovascular risk factors in CRPtg mice, a beneficial effect that was further enhanced by enrichment of the flavonoid conten